This invention relates to methods and apparatus for determining the polarity of a hydrophone, and more particularly, relates to methods and apparatus for determining the polarity of hydrophones mounted in marine seismic streamer cables.
A conventional hydrophone typically consists of a piezioelectric material mounted to allow the material to compress and dilate in response to a pressure field incident on the material. Wires attached to the exterior and interior surfaces of the material may be connected to external terminals of the hydrophone, and for one convention one terminal is marked with a dot of paint or a plus sign to indicate a positive polarity. However, many hydrophones may have color coded leads (blue and red) instead of external terminals. For either of such conventions, positive voltage is produced on the plus terminal or red colored lead when the hydrophone experiences a positive pressure, and the voltage is proportional to the relative strength of the pressure.
In seismic exploration the use of an array of hydrophones in a seismic cable to record the pressure fluctuations at various survey locations resulting from water-borne pressure sources is customary. The hydrophones may be connected in series, parallel, or a combination of series and parallel, depending upon acquisition factors. Correct polarity of each hydrophone is important so that the detector array response is the true sum of all the hydrophone elements. Detection of individual hydrophone polarities once assembled in a seismic streamer cable, i.e. an array, is at best a difficult problem.
Traditionally, hydrophone polarity has been determined with the use of a zero center scale galvanometer (or a microammeter) in an enclosed facility used to assemble or repair hydrophone cables. A hydrophone is connected to the galvanometer. When the hydrophone's exterior surface is pulsed by a pressure wave, the galvanometer deflects either plus or minus. A positive deflection of the galvanometer should occur if the hydrophone plus terminal or lead is connected to the plus terminal of the galvanometer. If the positive hydrophone terminal or lead is incorrectly marked, the galvanometer will have a negative deflection. Also, if the positive hydrophone terminal or lead is incorrectly marked the marking must be changed to reflect the actual positive terminal or lead.
Further, an oscilloscope may be used to replace the galvanometer and the transient displacement of the electron beam (i.e. output voltage of the hydrophone) indicates the polarity when a hydrophone is impulsed. If a digital storage oscilloscope or other recording device is used, then the transient deflection can be stored for later analysis. However, the use of oscilloscopes in the field (e.g. on a boat deck) to check the polarities of hydrophones installed in cables is not very practical due to the size and bulk of an oscilloscope, as well as its need for power.
Both of these methods require extreme care in handling, a lot of judgement, and attention to detail by the observer to determine the proper polarity and are not readily amenable to use in the field to test individual hydrophones, either uninstalled or installed in a cable. Thus, testing of hydrophones for polarity once installed in streamer cables for field acquisition is not practical using these methods. In addition, the testing of individual hydrophones for polarity before installation in a cable is a tedious procedure requiring attention to detail, as previously noted herein.
These and other limitations and disadvantages of the prior art are overcome by the present invention, however, and methods and apparatus are provided for determining and/or checking a hydrophone for proper polarity.